1. Field of the Invention
The present invention generally relates to a scanning speed modulating circuit for a television receiver. More particularly, the invention concerns a scanning speed modulating circuit for enhancing sharpness of a picture or image produced by a television receiver through modulation of the horizontal scanning speed of an electron beam.
2. Description of the Prior Art
As a system for enhancing sharpness of a picture image produced by a television receiver, a so-called aperture correction scheme is most commonly adopted in which contour portions of a picture or image are emphasized by making use of preshoot and overshoot. However, because the maximum beam current is increased at the contour of the picture in the case of the aperture correction scheme, spot diameter of the electron beam is increased with the width of the shoots being correspondingly enlarged resulting thus in slow or gentle change in brightness.
As another method of improving the sharpness of picture, there has been known a method of controlling the scanning speed of the electron beam of the television receiver in accordance with a video signal component. This known method will briefly reviewed below by referring to FIGS. 1A to 1D of the accompanying drawings.
FIG. 1A shows a pulse-like waveform as an example of a luminance signal. By differentiating this waveform by a differentiating circuit, there can be obtained a waveform illustrated in FIG. 1B. By utilizing this differentiated signal, the scanning speed of the electron beam in a CRT (cathode-ray tube) or Braun tube is modulated in a manner illustrated in FIG. 1C. As the result, brightness is decreased in an earlier half of the rise-up edge of the luminance signal because of a increased scanning speed, while in a later half of the rise-up edge of the luminance signal, brightness is increased because of the correspondingly decreased scanning speed. Similar phenomenon or effect takes place in relation to the falling edge of the luminance signal. Consequently, contour of a picture produced on the CRT screen is emphasized in a manner illustrated in FIG. 1D, whereby a picture whose sharpness is enhanced in appearance can be obtained.
As a practical method of modulating the horizontal scanning speed, it is known that an auxiliarly deflecting coil for modulating the scanning speed is provided at a neck portion of the CRT in addition to an ordinary deflecting coil system, wherein the differentiated signal current shown in FIG. 1B is caused to flow through the auxiliary deflecting coil (reference may be made to, for example, Japanese Patent Application Laid-Open No. 13064/1981).
The modulating method in which the auxiliary deflecting coil is employed, however, has drawbacks mentioned below.
First, the auxiliary deflecting coil is expensive. Second, when the auxiliary deflecting coil is to be mounted as a separate unit in addition to the ordinary deflecting coil system, a delicate and precise adjustment of the angle at which the auxiliary coil is mounted is required.
Besides, since the auxiliary coil is positioned as superposed on an electron gun, a focussing electric field of an electrostatic focussing lens is disturbed to exert adverse influence to the electron beam focussing performance with the result that astigmatism is likely to make appearance, to a serious disadvantage. In order to improve the sharpness, it is necessary that the beam spot is maintained in a small diameter. If the spot diameter is increased, there may arise a possibility that the intended purpose can not be attained. Furthermore, inductance of both the ordinary deflecting coil and the auxiliary deflecting coil will bring about various disturbances due to mutual interference of these coils, giving rise to, for example, problems that the sensitivity of the ordinary deflecting coil is degraded and misconvergence is undesirably produced.
As another practical method of modulating the horizontal scanning speed, it is known that a focussing electrode which the electron beam crosses at a single point within the electron gun is divided into two portions along a plane inclined relative to the center axis, wherein a scanning speed modulating signal is applied across the divided electrodes to thereby deflecting electrostatically the electron beam in the horizontal direction (reference may be made to, for example, IEEE Transactions on Consumer, February 1976, Vol. CE-22, No. 1, p.p. 13-21).
Although the above method can be applied to a one-gun type Braun tube such as a black and white CRT, trinitron type CRT or the like in which a single electron gun is used, a great difficulty will be encountered in adopting this method in a shadow-mask type color CRT in which three electron guns are employed, because of the great distance existing among the electron beams.
The method mentioned above can be applied to the CRT in which the single electron gun is used and the electron beam is concentrated at a single point within the electron gun. However, in view of the facts that the scanning speed modulating signal has to be applied to the focussing electrode to which a high voltage is applied, that restricting conditions are imposed on the design of the focussing electrode and that the focussing voltage is subjected to the influence of modulation of the electron beam scanning speed, it is difficult to realize the modulation of the scanning speed in the optimum spot state while maintaining the focussing voltage at an optimum level.